Radiotelephones, which are well known in the art, generally refer to communications terminals which can provide a wireless communications link to one or more other communications terminals. Such radiotelephones are used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems.
Essentially all radiotelephones include some type of antenna system for transmitting and/or receiving communications signals. Historically, monopole and dipole antennas have perhaps been most widely employed in various radiotelephone applications, due to their simplicity, wideband response, broad radiation pattern, and low cost. In particular, half-wavelength (.lambda./2) monopole and dipole antennas have been successfully employed in a large number of radiotelephone applications. However, as discussed below, such antennas simply are not suitable for certain radiotelephone applications.
As communications technology has matured, it has been possible to dramatically decrease the size of most radiotelephones, such that now many current radiotelephone applications are designed for mobile users who require small, handheld radiotelephones which are easily portable and which preferably fit conveniently within a user's pocket. However, traditional half-wavelength and quarter-wavelength monopole antennas are not well-suited for such applications, as the large size of these antennas with respect to the relatively small size of modern handheld transceivers makes such antennas impracticably large for use on such a handheld radiotelephone.
Helix antennas represent one potential solution to the size problem associated with monopole antennas in handheld radiotelephone applications. This class of antenna refers to antennas which comprise a conducting member wound in a helical pattern. As the conducting member is wound about an axis, the axial length of a quarter-wavelength or half-wavelength helix antenna is considerably less than the length of a comparable quarter-wavelength monopole antenna, and thus helix antennas may often be employed where the length of a quarter-wavelength monopole antenna is prohibitive. Moreover, although a half-wavelength or a quarter-wavelength helix antenna is typically considerably shorter than its half-wavelength or quarter-wavelength monopole antenna counterpart, it may exhibit the same effective electrical length.
Another advantage associated with helix antennas which makes them well-suited for many radiotelephone applications is their design flexibility. For instance, helix antennas may be designed to operate in several modes, each of which provides a different type of radiation pattern. One such mode is referred to as the "axial mode" of operation, which typically may be achieved by designing the helix antenna to have an axial length several times larger than the wavelength corresponding to the intended frequency of operation. In this mode, the helix antenna typically provides a relatively high gain radiation pattern, and this pattern may be maintained over a relatively large operating bandwidth. However, the radiation pattern provided in axial mode is highly directional and circularly polarized and hence axial mode operation is typically not appropriate for mobile radiotelephone applications, such as cellular telephone, in which the user held handsets do not track the base station antennas.
A second mode in which helix antennas may operate is referred to as normal mode. To operate in this mode, a helix antenna typically has a radiating element of resonant length (i.e., 1/4.lambda., 1/2.lambda., 3/4.lambda. or .lambda. in length, where .lambda. is the wavelength corresponding to the center frequency of the frequency band over which the antenna is to operate) that is wound on a small diameter with a small pitch angle. Thus helix antennas which are designed to operate in normal mode are conveniently small and well-suited for various portable radiotelephone applications such as cellular telephone. In normal mode, the antenna typically provides a linearly polarized doughnut-shaped radiation pattern which is also well-suited for cellular telephone applications, but unfortunately, the antenna only provides this radiation pattern over a relatively narrow bandwidth situated about the resonant frequency. Moreover, the natural bandwidth of the antenna is proportional to the diameter of the cylinder defined by the helically wound radiating element of the antenna, and thus, all else being equal, the smaller the diameter of the antenna, the smaller the operating bandwidth.
While helix antennas, operated in either axial mode, normal mode or a proportional combination of the two, are a logical choice in many applications where a more traditional dipole or monopole antenna is too large, there are a number of radiotelephone applications which require a relatively small antenna, that is capable of transmitting and/or receiving signals in two or more widely separated frequency bands. One example application is dual-band cellular telephones, which refer to cellular phones which operate in two frequency bands, such as the 850 MHz and 1920 MHz frequency bands. Various satellite communications systems provide another example of applications requiring dual-band capability, as such systems typically have widely separated transmit and receive frequency bands. Unfortunately, however, as discussed above, helix antennas generally are not well-suited for these applications, as they typically are incapable of providing a quasi-omni-directional radiation pattern over a wide band of frequencies due to the potential bandwidth limitations of this type of antenna when operated in normal mode.
Despite the above-mentioned limitations of helix antennas, several dual-band helix antenna systems have been proposed. For instance, U.S. Pat. No. 4,554,554 to Olesen et al. discusses a quadrifilar helix antenna which includes PIN diode switches along each of its elements to provide means for selectively resonating the antenna at one of two distinct frequencies by changing the electrical length of the elements. However, the antenna disclosed in Olesen et al. does not solve the above-mentioned problem as it operates in axial mode, and hence does not provide an omni-directional radiation pattern, and any corresponding design of the antenna to operate in normal mode may be impractically large for handheld radiotelephones.
Similarly, U.S. Pat. No. 4,494,122 to Garay et al. discusses an antenna system comprising an upper radiating element and a tank circuit which resonate at one frequency, and a helical element and associated sleeve member which resonate at a second frequency. While this apparatus is potentially shorter than a conventional sleeved dipole, it is still relatively large, and the usable operating bandwidth of the antenna about each resonant frequency is very small, such that this antenna system is not suitable for many potential dual-band applications such as cellular telephone.
U.S. Pat. No. 4,442,438 to Siwiak et al. discusses an antenna system comprising two quarter-wavelength helical antenna elements and a linear conductive member, which purportedly resonates at two different frequencies. However, the antenna disclosed in Siwiak et al. does not resonate at widely separated frequencies (the resonant frequencies disclosed were 827 MHz and 850 MHz), as the antenna is designed to broaden the antennas response to cover a single bandwidth of operation as opposed to providing for operation in two widely separated frequency bands.
Finally, additional helix antenna systems are disclosed in Japanese Pat. No. 5-136623 and U.S. patent application Ser. No. 08-725507, which discuss dual band operation through use of a conductive tube, and variable pitch windings, respectively. However, the mechanism for providing dual-band operation used in both these approaches, namely coupling between adjacent windings on the helix, typically results in a narrow operating bandwidth in the higher of the frequency bands and further may provide only limited design flexibility. Moreover, the antenna discussed in Japanese Patent No. 5-136623 also has a reduced effective aperture in the higher of the frequency bands.
Thus, in light of the above-mentioned demand for dual-band radiotelephones and the problems with current antenna systems for such radiotelephones, a need exists for small, omni-directional radiotelephone antenna systems that are capable of operating in two widely separated frequency bands.